Para-xylene is oxidized to produce terephthalic acid or dimethyl terephthalate which are polymerized to yield polyester fibers and films. Processes for the production of terephthalic acid (1,4-benzenedicarboxylic acid) and dimethyl terephthalate are well known and described, e.g., in U.S. Pat. No. 2,833,816 and British Patent Specification Nos. 809,730 and 727,989.
Of the three xylene isomers, meta, ortho and para, only p-xylene is suitable for the production of terephthalic acid and/or dimethyl terephthalate for polyester manufacture because of the ability of p-xylene to form straight polymer chains. Straight claims are necessary to give polyester its fiber-forming or film-forming characteristics and high tensile strength.
The p-xylene must be substantially pure to avoid unwanted side reactions when the p-xylene is oxidized to terephthalic acid.
Para-xylene has been typically produced by methylation of toluene, e.g., by reaction of toluene with methanol as generally described by Chen et al., J. Am. Chem. Soc. 1979, 101:6783, and by toluene disproportionation, e.g., as generally described by Pines in The Chemistry of Catalytic Hydrocarbon Conversions, Academic Press, New York 1981, p. 72. Such methods typically produce a mixture of C.sub.8 products including para-xylene, ortho-xylene, meta-xylene and ethylbenzene. Para-xylene may be recovered from mixed C.sub.8 streams followed by xylene isomerization of the remaining stream as described, e.g., in U.S. Pat. Nos. 3,856,871 and Re 31,782.
In terephthalic acid production, one unwanted by-product results from the presence of o-xylene in the xylene feed. Any o-xylene present is oxidized to orthophthalic acid and subsequently dehydrated to phthalic anhydride, interfering with terephthalic acid production and impacting on the oxidation catalyst lifetime. If ethylbenzene (EB), another C.sub.8, is present in the p-xylene feed for terephthalic and production, the EB may be oxidized to benzoic acid. Meta-xylene is oxidized to isophthalic acid. These oxidation products are more easily separated from PTA than the oxidation products of o-xylene. The quality of polyester is affected by even small amounts of organic or inorganic impurities which cause process difficulties during polymerization and affect the color, thermal and photochemical stability, and long-term durability of the polyester product. The precursor for polyester production is purified terephthalic acid (PTA). Further, the PTA thus produced may be esterified with an alcohol, e.g., reaction of PTA with methanol yields dimethyl terephthalate which may also be used in polyester production.
As disclosed, e.g., in British Patent Specification No. 727,989, if the initial p-xylene feed does not consist of pure para-xylene compound but also contains other xylene isomers and/or non-aromatic compounds, a purification of the feed is necessary. Purification of the p-xylene feed is generally carried out using physical processes such as fractional distillation or crystallization described, e.g., in U.S. Pat. Nos. 3,177,255 and 3,467,724, absorption described, e.g., in U.S. Pat. No. 2,985,589, or chemical processes, e.g., contacting with formaldehyde in acid solution which removes m-xylene as a resin. A widely used method is UOP's PAREX.RTM. process described by J. J. Jeanneret, "UOP Parex Process," in Handbook of Petroleum Refining Processes, R. A. Meyers, ed., McGraw - Hill, New York 1997. Other methods for separating p-xylene from a C.sub.8 aromatic mixture are described by U.S. Pat. No. 4,705,909 and references cited therein. In conventional xylene manufacture, significant costs are incurred for p-xylene purification for PTA production.
It is an object of the invention to improve terephthalic acid and/or dimethyl terephthate manufacture by minimizing unwanted oxidation by-products. It is another object of the invention to eliminate the need for a purification process for p-xylene to be used in terephthalic acid and/or dimethyl terephthalate production.